1. Technical Field
The invention relates generally to a method and apparatus for wellbore coring and logging. More particularly, this invention relates to a method and apparatus for collecting data regarding geological properties of underground or undersea formations during coring operations.
2. Discussion of Related Art
The desirability of a system which is able to measure downhole formation properties while simultaneously coring a geological sample has long been recognized. Until now it has not been possible to continuously collect large diameter core and in situ logging data simultaneously.
Geologists and geophysicists collect data regarding underground formations in order to predict the location of hydrocarbons (e.g., oil and gas). Traditionally, such information is gathered during an exploration phase. In recent years, however, the art has advanced to allow the collection of geophysical and geological data as a well is being drilled. These logging-while-drilling (LWD) measurements are typically made following coring in a separate borehole. Logging data are correlated to the core sample. Correlation accuracy depends on the yield recovery of the core and sample/data match-up. There is a pressing need in the industry for more accurate formation property data, such as provided by correlation of the core to a downhole data set.
Known systems (e.g., logging-while-drilling) use a series of tubes, referred to as drill pipe and collars, to drill a hole into the formation. The lower end of the drill string, called the bottomhole assembly, is provided with a cutting mechanism, referred to as drill bit, which has a concentric hole. A drill collar, disposed proximally to the drill bit, includes several formation properties sensors, referred to as an LWD tool. Formation property measurements are recorded in this LWD tool.
When a sample of the formation is required, a coring device is lowered inside the drill string and secured at the bottom end. By resuming drilling and/or pumping fluid down the drill string, the coring process is effected. The coring device is retrieved by a latching mechanism attached to a wireline.
Continuous wireline-retrievable coring, for example, is routine in nearly all Ocean Drilling program (ODP) drill holes, whereas industry coring programs are often limited in key intervals due to time and cost constraints. The ODP routinely drills holes up to 2000 m deep without a riser in water depths ranging from 300 m to 6000 m. Sea water is utilized at high pressure to clear the hole of cuttings. Conventional wireline logging tools are typically deployed if hole conditions are good. In cases where drilling is expected to be difficult, LWD technologies are employed in another hole in close proximity to the core hole. A dedicated LWD hole is often the only alternative to collect in situ log data in such difficult drilling environments.
In order to obtain logging-while-drilling data and a closely correlated core sample, the prior art requires two holes to be drilled. A first hole is drilled to collect a core sample. A coring bottomhole assembly is used to simultaneously drill a hole and core out a core column. A second hole, laterally spaced from the first hole, is drilled using a traditional logging-while-drilling bottomhole assembly. Logging-while-drilling tools measure formation properties of borehole that are, in theory, supposed to be closely correlated to the previously extracted core sample.
The prior art exhibits two significant disadvantages. The above described method is time consuming because it requires two separate drill holes: a first hole for obtaining core samples and a second hole for obtaining logging-while-drilling data. Specifically, a downhole coring assembly must be lowered to the ocean floor, in order to drill/core the first hole. Subsequently, the downhole coring assembly is raised to the surface so that a retooling can be executed. A logging-while-drilling downhole assembly is then lowered back down to the ocean floor in the area of the first hole. Following the positioning of the logging-while-drilling downhole assembly, the assembly drills the second hole while performing logging-while-drilling measurements. The time required in refitting the drillstring with the logging-while-drilling assembly and in drill the second hole adds to the total operating costs and time duration of this coring and logging operation.
The second disadvantage is the possible detrimental effect on the data correlation. Correlating a core sample with formation property data assumes that the data and sample are obtained from same location or even the same hole. When the logging data and core sample are obtained from different holes that are often located some distance from each other, one's ability to correlate the logging data with the core sample to obtain accurate result can be adversely affected.